This invention relates to a mixer for use in an AM-FM radio receiver and particularly to such a mixer adapted for use in an integrated circuit in which there is as much integration as possible between the AM and the FM circuitry.
In some radios, a significant amount of common circuitry is possible in the AM and FM signal paths. For example, a dual conversion AM-FM radio receiver of the type wherein the first VCO converts AM signals up and FM signals down to a common first IF frequency of 10.7 MHz may include a common first IF section and a common second IF section at 450 KHz. The mixer which converts the first intermediate frequency signal down to the second intermediate frequency signal appears between the first and second IF sections and ideally would be common to AM and FM signal paths. However, the output of this mixer must, for AM signals, pass through a relatively narrow ceramic bandpass filter at 450 KHz; whereas the FM signal output needs a wider passband. Thus it is desired to provide a single mixer between the first and second IF sections which allows separate output paths for AM and FM signals.